1. Field of the Invention
The present invention relates to a data flow amount control device and a data flow amount control method.
2. Description of the Related Art
In mobile communication systems such as cellular phones which have been widely used, entire service area is divided into a plurality of radio zones which are referred to as cells and services are provided in the cells. As shown in FIG. 1, such a mobile communication system includes a plurality of base stations covering the cells, a plurality of mobile stations which perform communications by setting radio channels between the mobile stations and the base stations, and control stations which control the plurality of base stations and the plurality of mobile stations. Further, the control stations are connected to a core network including an exchange and the like.
In such a configuration, signal transmission in the communications between the core network and the mobile station is performed through the control station and the base station. Thus, a base station performs relaying between a wired section and a radio section, the wired section being established between a core network and a base station via a control station, and the radio section being established between a base station and a mobile station. The base station has a function of temporarily buffering a signal transmitted from the core network and control station. An object of the buffering function is to buffer a signal from the wired section when a transmission rate in the wired section is greater than that in the radio section.
Meanwhile, a high speed downlink packet transmission, which is of high-speed and large-volume, such as High Speed Downlink Packet Access (HSDPA) or 1x EV-DO is standardized, and also has been developed (for example, 3GPP TR25.848 v4.0.0 and 3GPP2 C. s0024 Rev. 1.0.0).
A main characteristic of such a high speed packet transmission scheme is to utilize an adaptive modulation and coding scheme in which packet transmission formats are adaptively changed according to radio conditions of a mobile station which performs a packet transmission. Here, in such a high speed packet transmission scheme, the packet transmission formats are a data size, a modulation scheme, the number of codes, a coding rate of packets and the like.
In this adaptive modulation and coding scheme, a radio transmission data rate changes with time according to a radio condition of a mobile station. For example, in the HSDPA, the data size of a packet, a modulation scheme, and the number of codes are controlled according to a radio condition between a mobile station and a base station, whereby an adaptive modulation and coding is performed.
In addition, by use of the adaptive modulation and coding scheme in above HSDPA and 1x EV-DO, packets are transmitted through a radio section in such a transmission format that an error rate of the packets would be a predetermined value. However, the actual error rate varies according to a propagation environment of the radio section including, for example, a moving speed of the mobile station, the number of paths and the number of base stations performing communications at the same time. For example, in the case of HSDPA, packets are transmitted through the radio section in a transmission format having an error rate of 10%. However, the actual error rate is 10% to 40%. Thus, the radio transmission data rate changes from time to time according to the error rate in the radio section.
By the way, in a case of the mobile communication system configured as described above, when the transmission data rate of the wired section is larger than that of the radio section, packets tend to be stored in a temporal buffer of a relaying base station; and, when the transmission data rate of the wired section is smaller than that of the radio section, packets tend to be exhausted in the temporal buffer of the relaying base station.
Due to the limitation of the amount of the packets stored in the temporal buffer of the relaying base station, the packets overflowed from the temporal buffer are deleted. In addition, due to the exhaustion of the packets stored in the buffer of the relaying base station, an entire radio bandwidth cannot be always used up.
Accordingly, when there is a difference between a transmission data rate in a radio section and a transmission data rate in a wired section, a transmission efficiency is more likely to deteriorate due to the deletion of packets caused by a buffer overflow, or an utilization efficiency of a radio transmission bandwidth is more likely to deteriorate due to the exhaustion of the buffer.
In order to avoid the above two problems, it is necessary to control the transmission data rate in the radio section and the transmission data rate in the wired section so that these transmission data rates can be made as equal as possible.
From the viewpoint of suppressing the deletion of packets in the base station in communications between the core network and the mobile station, a method has been proposed in which a data flow amount control for controlling an amount of packets transmitted from a control station to a base station is performed according to an amount of packets stored in the buffer of the base station (Japanese Patent Application Laid-open Publication No. 2002-077987).
Moreover, another method (Japanese Patent Application Laid-open Publication No. 2005-057323) has been also proposed for such a communication system in which a radio transmission data rate greatly changes like a high speed packet communication scheme, and this method makes it possible to prevent the deterioration of the utilization efficiency of the radio transmission bandwidth and the deterioration of the transmission efficiency of the communication system. Here, the deterioration of the utilization efficiency might be caused by the exhaustion of the buffer and the deterioration of the transmission efficiency might be caused by a retransmission control due to the deletion of packets by a buffer overflow.
By using this scheme, the amount of data flow from the control station to the base station can be controlled according to the amount of packets stored in the buffer, so that a buffer overflow in the base station can be avoided.